Mobile, wireless, and/or handheld portable devices increasingly become multifunctional communication devices. These handheld portable devices integrate an increasingly wide range of functions for handling a plurality of wireless communication services. For example, a single handheld portable device may include an FM receiver along with GPS, CDMA, Wi-Fi, WiMAX, CDMA2000 and 3G receivers.
FM receivers require wideband low-noise amplifiers (LNAs) as they operate from 76 MHz to 108 MHz. Specifications requirements for FM receiver sensitivity include a wide band LNA with sufficient gain and a noise figure (NF) that is below 3 dB. This NF might be easily achievable for LNAs using source degenerating inductors.
Moreover, source impedance matching is usually required to limit reflections on an antenna or to avoid alterations of the characteristics of any RF filter preceding the LNA. An LNA exploiting an inductor achieves such requirements, but only in a narrow frequency band around resonance.
Inductors tend to occupy significant die area. As in newer CMOS technologies the area costs increase, area consuming inductors increase the overall device cost. FM receivers are considered auxiliary products in a multifunctional communication device and they must occupy very small die area so they do not impact the overall device cost. Thus, the use of inductors for FM receivers is prohibitive.
Alternative solutions for impedance matching have to be considered to achieve power matching at the input of the LNA. Alternative solutions for impedance matching have to be considered to achieve power matching at the input of the LNA. One such solution may include adding input resistive termination to a common source gain stage of the LNA. An alternative solution is to use a common gate as input stage of the LNA. Yet another solution is to use a shunt feedback common source stage as input stage of the LNA. These methods would provide a good power matching but may greatly degrade the NF. Most LNAs based on resistive termination provide good power matching but greatly degrade the NF.
To decouple impedance matching from noise figure various circuit techniques that include noise canceling have been proposed. Most of these circuit techniques are based on the following principle of operation: An RF signal appears at a first node while a replica of the RF signal, proportional to the RF signal, appears at a second node. However, the thermal noise contribution due to the input stage of the LNA appears with opposite polarity at each node. Thus, noise cancellation may be achieved by summing the RF signal and its replica.
Various such topologies have been shown in the literature. Yet, there is always a need to improve them and achieve even smaller die size and even lower power consumption, particularly for auxiliary components such as FM receivers.
FIG. 1 shows an implementation of an LNA with noise canceling for an FM receiver.
The LNA includes input stage circuit 12, first amplifier 14 and second amplifier 16. Input stage circuit 12, includes MOS transistor M1 in a common gate configuration coupled to load resistor RL. First amplifier 14 comprises a pair of complimentary MOS transistors, M2p and M2n, in a common source configuration and is ac-coupled to the second amplifier through capacitors C3ac and C4ac. Each of these capacitors along with each of resistors R1DG and R2DG, respectively, forms a high-pass filter for the signal appearing at the output of first amplifier 14. Typically, R1DG is equal to R2DG. The impedance at the input of the second amplifier, and more specifically at the source node of transistors M3n and M3p, is 1/gm in parallel with R1DG or R2DG. The transconductance of transistors M3n and M3p is defined as gm. Given reasonable transistor size and biasing current, the impedance looking into the input of the second stage may be well below 1K ohm. Within the FM band, (76 MHz˜108 MHz), the signal at the output of the first amplifier has to pass through the high-pass filter with minimum loss, which dictates that the high pass corner of the high-pass filter has to be much lower than the lower operating frequency of the FM band (76 MHz). Thus, ac coupling capacitors C3ac and C4ac have to be quite large to meet this requirement.
The use of large capacitors is undesirable not only because they occupy significant die area but also because they do not allow sharing or reusing of current between the different amplification stages of the LNA; thus, resulting in noise canceling solutions with a high current consumption LNA.
It is desirable to design a noise canceling LNA that does not require either a large inductor or large capacitors and that is suitable for low current consumption.